1. Field of the Invention
The present invention relates to solar power production, and more particularly to systems and methods of power production with solar boilers.
2. Description of Related Art
Solar power generation has been considered a viable source to help provide for energy needs in a time of increasing consciousness of the environmental aspects of power production. Solar energy production relies mainly on the ability to collect and convert energy freely available from the sun and can be produced with very little impact on the environment. Solar power can be produced without creating radioactive waste as in nuclear power production, and without producing pollutant emissions including greenhouse gases as in fossil fuel power production. Solar power production is independent of fluctuating fuel costs and does not consume non-renewable resources.
Solar power generators generally employ fields of controlled mirrors, called heliostats, to gather and concentrate sunlight on a receiver to provide a heat source for power production. A solar receiver typically takes the form of a panel of tubes conveying a working fluid therethrough. Previous solar generators have used working fluids such as molten salt because it has the ability to store energy, allowing power generation when there is little or no solar radiation, such as at night time. The heated working fluids are typically conveyed to a heat exchanger where they release heat into a second working fluid such as air, water, or steam. Power is generated by driving heated air or steam through a turbine that drives an electrical generator.
More recently, it has been determined that solar production can be increased and simplified by using water/steam as the only working fluid in a receiver that is a boiler. This can eliminate the need for an inefficient heat exchanger between two different working fluids. This development has lead to new challenges in handling the intense solar heat without damage to the system. In a solar boiler, heat transfer rates can reach levels around 2-3 times the heat transfer rate of a typical fossil fuel fired boiler. This high heat transfer rate intensifies problems related to maintaining even heating and flow distribution throughout known designs of boiler panels. If flow through a portion of a receiver panel is insufficient when using water/steam as a working fluid, overheating can result for that panel portion. Such overheating can result in damage or failure of the panel and its constituent tubes if the temperatures are allowed to become severe.
Another way in which solar boilers differ from fossil fuel fired boilers is in terms of operating hours. Fossil fuel fired boilers typically operate continuously, stopping only for occasional routine maintenance, whereas solar boilers must start up and shut down once per day due to the rising and setting of the sun. Any working fluid and boiler components that cool down during the overnight layover must be brought back up to operating temperature each morning. There results a daily thermal expansion cycle that can result in increased fatigue failure in typical boiler components.
Traditional boiler designs use considerable time in starting up and shutting down. Since solar power is only available during daylight hours each day, these lengthy start up and shut down cycles can deprive the system of valuable operating hours that could otherwise be used for power production. One reason for the lengthy start up and shut down times is the difference in day time operating temperature and layover temperature at night, which can reach freezing temperatures. For example, starting a traditional boiler system too rapidly under these conditions can exacerbate the thermal expansion issues described above. On the other hand, starting a traditional boiler system too slowly can result in a failure to adequately circulate fluids through the boiler tubes, which can lead to boiler tube failure under the intense heat flux.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still an need in the art for systems and methods that allow for improved startup and shut down of solar boilers. There also remains a need in the art for such systems and methods that are easy to make and use. The present invention provides a solution for these problems.